Difficulties arise with high frequency radar when range and velocity are both to be measured, and when either the radar or target has substantial range or velocity. This is the so-called range-Doppler ambiguity. For example, for measurements from an earth orbiting platform at an orbital altitude of about 800 km, the platform velocities are about 7kms.sup.-1, and the two-way time delay for echoes directly beneath the satellite and near the earth's surface will be about 5 ms; however, for a radar transmitting at a frequency of 10,000 MHz, the Doppler shift of a return measured from a radar on the orbiting platform would be on the order of 0.5 MHz. The maximum unambiguous frequency measured by this radar with a pulse-repetition-frequency (PRF) equal to the inverse of the 5 ms ranging time would be 100 Hz. Thus the PRF needed for ranging cannot fully sample the Doppler shift. In radar parlance, the velocity information is "aliased" and ambiguous. Also, since the radar antenna beamwidth causes spreading of the width of the Doppler spectrum, it is possible for this spread to equal or exceed the spacing between Doppler ambiguities, making velocity measurement difficult or impossible.
An example of the need for ambiguity resolution is the case of a 100 GHz wind-velocity sensing satellite radar. Assuming that nadir and forward-looking measurements are needed to sense the velocity components, then for a 30 degree forward angle, the Doppler shift for a 7 km/sec ground-track velocity is about 2.3 MHz. The Doppler bandwidth for a 0.8M antenna would be about 20 kHz. Therefore, for any reasonable PRF, the felocity data is heavily aliased and the spectral expanse of the signal is comparable to the Nyquist interval.